1. Field of the Invention
This invention relates to a logic circuit and more particularly to a logic circuit suitable for the bipolar-CMOS (biCMOS) IC consisting of a CMOS transistor circuit for performing logic operation with input signals and a bipolar transistor circuit constructed as an output circuit for outputting the result of the operation.
2. Description of the Prior Art
Logic circuits of such type have combined advantage of low power dissipation involving the CMOS transistor circuit and large output current and high-speed operation involving the bipolar transistor circuit, and thus their use has become widespread.
Such a logic circuit comprises a first bipolar transistor for the signal output, of which the collector is connected to the first power supply terminal having a specified potential and the emitter is connected to the output terminal; a first MOS transistor circuit which consists of a p-(or n-)channel type MOS transistor, for making or breaking connection between the collector and the base of the first bipolar transistor in response of the result of a specified logic operation with the input signal; a second bipolar transistor for the signal output, of which the collector is connected to the output terminal and the emitter is connected to the second power supply terminal being at ground potential; and a second MOS transistor circuit, which consists of a n-(or p-)channel type MOS transistor, for making or breaking connection between the collector and the base of the second bipolar transistor in response of the result of a specified logic operation with input signals and in the reverse relation to the first bipolar transistor. The above output terminal is usually connected through a connection line to the next-stage circuit, and due to this connection line capacitance and the MOS transistor gate capacitances, the load circuit of this logic circuit is capacitive.
Once the first MOS transistor circuit is turned ON in response to the level change of the input signal, electric current flows to the base of the first bipolar transistor, the latter turning ON, and consequently the collector-emitter current which is several and tens times more than the base current flows. Under such conditions, the second MOS transistor circuit is turned OFF, which turns the second bipolar transistor OFF. Thus the load circuit is rapidly charged and its terminal voltage gets near the above-mentioned supply potential.
On the other hand when the first MOS transistor circuit turns OFF and the n-channel MOS transistor circuit turns ON, electric current flows to the base of the second bipolar transistor from the load circuit and thus the collector-emitter current of several and several tens times more than the base current flows to the second bipolar transistor, thereby the load circuit being rapidly discharged, and thus the terminal voltage get near ground potential.
The bipolar transistor however keeps ON-state, after the base current has been shut off, until completion of the base discharge. Once the first MOS transistor circuit turns ON, and in turn the load circuit begins to be charged. For a while after that, the second bipolar transistor is kept in ON state, and hence a part of current bypasses the load circuit by passing through the second bipolar transistor, resulting not only in longer time required for charging the load circuit but also in increased power consumption. Similarly bypass through the first bipolar transistor during charging the load circuit through the second bipolar transistor causes prolongation of the time required for the charging and increase of power consumption.
Accordingly there is provided in such kind of logic circuit a base-discharge circuit for each of the first and second bipolar transistors. Base-discharge circuits falls into two types of resistor circuit and MOS transistor circuit.
In the case of the resistor circuit type, a first resistor is connected between the base and the emitter of the first bipolar transistor and a second resistor between the second bipolar transistor of the base and emitter. These first and second resistors facilitate the discharge of the corresponding base of the bipolar transistors to make turn-OFF of the bipolar transistors faster, thereby increase in charging/discharging speed of the load circuit, and reduction of power consumption is attempted.
Fall in resistance value for hastening the base-discharge by this method however brings reduction of base current and slows turn-ON of bipolar transistor. Conversely increase in resistance value increases turn-ON speed and decreases turn-OFF. In summary it has been difficult to accelerate both turn-ON and turn-OFF.
On the other hand, in the latter case or with the base discharge circuit of the MOS transistor circuit type, a third MOS transistor circuit including n-(p-)channel type MOS transistor which turns ON or OFF in response to the result of logic operation with input signals is connected between the base of the first bipolar transistor and a ground potential point, and n-(or p-)channel type MOS transistor of which gate is connected to the output terminal and capable to turn ON or OFF in response to the signal level of the output terminal is connected between the base of the second bipolar transistor and a ground potential point. In this way, when the first MOS transistor circuit is turned OFF, which in turn shuts off the current to the base of the first bipolar transistor, then the third MOS transistor circuit is turned ON, and the base of the first bipolar transistor is discharged through the third MOS transistor circuit, which rapidly turns OFF the first bipolar transistor. Besides when the first MOS transistor circuit turns ON, the third MOS transistor circuit is turned OFF, and hence all the current through the first MOS transistor circuit is supplied to the base of the first bipolar transistor, and thus the first bipolar transistor is rapidly turned ON. Turn-ON and turn-OFF is in inverse relation between the first and third MOS transistor circuits. The electric currents flowing through MOS transistor circuits, respectively, decrease remarkably compared with in the former case with a resistor circuit type. The second bipolar transistor has similar relation to the second MOS transistor circuit and to the base-discharge MOS transistor, accordingly with higher effects of increasing speed and reducing power consumption than in the former case of the resistor circuit type.
The load circuit however is charged only with the emitter current of the first bipolar transistor, and hence the maximum effective voltage for charging the load circuit is lower by the base-emitter voltage of the first bipolar transistor. On the other hand, in the course of discharging the load circuit, its potential falls, and consequently the MOS transistor for discharging the base of the second bipolar transistor is turned OFF. Hence discharge thereafter is carried out only with emitter current of it. This implies that the minimum discharge potential of the load circuit is higher by the base-emitter voltage of the second bipolar transistor, correspondingly smaller amplitude of the output signal and lower noise margin results.
Besides the detailed investigation of the way that the first MOS transistor circuit and the third MOS transistor circuit are turned ON or OFF has revealed the following: between the MOS transistors which these MOS transistor circuits, respectively, consist of, there is difference in threshold voltage due to their different conductivity types. Also it takes some time for turning ON and OFF. Owing of these, their ON periods are overlapped as very short. Similar problem is encountered in the second MOS transistor circuit and the base-discharge MOS transistor. The latter approach cannot be said satisfactory and remains to be improved though it is of high-speed and low power consumption compared to the former.